This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which utilize both fundamental and harmonic ultrasonic signals for imaging.
In ultrasonic harmonic imaging, two dimensional (2D) or three dimensional (3D) images are formed by transmitting ultrasound at one frequency (or range of frequencies) and receiving at this frequency and higher harmonics of the transmit frequency. These harmonic signals are generated either by scattering from bubbles (harmonic contrast) as described in U.S. Pat. No. 5,879,303 or by non-linear propagation in tissue (tissue harmonic imaging, or THI) as described in U.S. Pat. No. 5,833,613. Typically, receive beams are formed only from the second harmonic echo signals, with the transmitted (or xe2x80x9cfundamentalxe2x80x9d) echo signals being removed either by filtering or by cancellation techniques such as pulse inversion. See U.S. Pat. No. 5,951,478. For THI, adequate removal of the fundamental signal is essential for the improvements in clutter suppression and contrast resolution which are typically seen.
In order to receive sufficient 2nd harmonic signal energy to form an high quality image, the transmitted signal frequency is typically lowered to approximately ⅔ of the xe2x80x9cnominalxe2x80x9d center frequency of the scanhead. This is illustrated in FIG. 1, where Tx illustrates a transmit band, Rx illustrates a 2nd harmonic receive band, and the passband 14 of the scanhead transducer has a nominal center frequency of Fc. This means that the 2nd harmonic signal is now at {fraction (4/3)} of the nominal center frequency and hence still within the bandwidth of a typical transducer. For example, for a nominally 3 MHz transducer center frequency operating in harmonic mode the transmit frequency might be 2 MHz with the 2nd harmonic at 4 MHz. Although the transmitted (or fundamental) frequencies can be partially removed from the received beam by filtering techniques, pulse inversion, in which two pulses of opposite polarity are transmitted sequentially and the rf echo signals received from each pulse are summed, is much more effective, at least in situations of comparatively slow target movement. The major disadvantage of pulse inversion is the factor of two decrease in frame rate that is incurred.
Under some circumstances it may be of interest to image with both the 2nd harmonic signal and the fundamental signal which would normally be discarded in harmonica imaging. For example, two of the limitations of THI are poor near-field imaging (before non-linear propagation has had a chance to generate a significant 2nd harmonic response) and poor penetration, because the 2nd harmonic echo is highly attenuated as compared to the fundamental echo signal. One option for addressing this problem is to image with the fundamental signal in the near and far fields while imaging the 2nd harmonic in the mid-field. See, for example, U.S. Pat. No. 6,283,919 which teaches the formation of ultrasonic images which are a blend of fundamental and harmonic signals. Fundamental signals can also be combined with 2nd harmonic signals to provide speckle reduction through frequency compounding.
One of the main disadvantages with imaging the fundamental signal from a typical harmonic imaging transmit burst is that the fundamental is typically of lower frequency and bandwidth than that used for xe2x80x9cconventionalxe2x80x9d fundamental-only imaging, as shown in FIG. 1, resulting in poor lateral and axial resolution. Thus, it would be desirable to transmit bursts which are optimized for both fundamental and harmonic performance to realize the full benefits of the respective imaging modes, and to do so with little or no penalty in frame rate.
In accordance with the principles of the present invention, a method is described for imaging both conventional fundamental and 2nd harmonic signals with little or no frame rate loss. In a preferred embodiment the inventive method includes transmitting an ultrasound wave which incorporates two transmit waveforms, one optimized for fundamental imaging and another optimized for THI imaging. In accordance with a further aspect of the present invention, two sequential transmit bursts of this form are used, with the same THI waveform but different fundamental waveforms. When the depth-corresponding echoes from the two bursts are summed, the optimized fundamental signals cancel, leaving a distinct and optimized THI echo and its transmitted fundamental, which may be processed in the usual manner for THI imaging. When the echoes from the two bursts are subtracted, the optimized fundamental signals are preserved as the THI signals are cancelled. These signals may be processed in the usual manner for fundamental imaging. Since the two signals are available simultaneously, images which are a blend of optimized fundamental and harmonic signals can be produced.